The present invention relates to the field of AM stereo broadcasting, and more particularly to a quadrature AM modulator for use in commercial radio broadcasting.
Commercial AM radio broadcasting in the United States has long been a monophonic medium. Recently, however, there has been substantial interest in introducing signal formats for broadcasting stereophonically in the AM broadcasting band. Numerous different signal formatting schemes have been proposed. Each scheme has tried to provide optimum stereo performance, while still providing a signal which could be received by existing monophonic AM receivers without objectional levels of distortion. In one system, described in U.S. Pat. No. 4,225,751, a quadrature AM signal is transmitted, however the gain of the quadrature channel signal is dynamically varied in accordance with an estimation of the amount of distortion which the signal will cause in a conventional monophonic receiver. A pilot signal is modulated in accordance with the varying gain of the quadrature channel signal and is transmitted as part of the quadrature signal. A stereo receiver demodulates the pilot signal to recover a gain control signal, and uses the gain control signal to adjust the gain of the quadrature channel signal so as to compensate for the varying gain introduced at the transmitter.